I'll start! WHAT IF we could use an underwater microphone (called a hydrophone). One for the ROV, and one for the remote control. Then, we could create customized and unique "whale songs" that the ROV could convert into movement commands. i.e., To descend for 5 seconds, we could send a series of pitches (not easily replicated in nature, of course!...we don't want the dolphins to take control of the ROV!).
Of course, there are some bumps to work out. We'd have to test the effective distance, as well as the potential for background noise interference from the environment or from the drone itself.